Flexible couplings are used for example in drive trains where a certain amount of flex needs to be accommodated. For example in the drive trains for flaps and slats on aircraft wings, a certain amount of flex in the wings needs to be accommodated.
One type of flexible coupling that has been extensively used is a barreled spline joint. The barreling on this joint allows angular movement of one shaft with respect to the other. However, excessive angular deviation can lead to compromise of the environmental seal of the joint. In tests on one barreled spline joint a torque of less than 50 Nm was required before the joint reached an angle of 7 degrees at which point the seal is considered compromised. At a bending moment of less than 100 Nm, the barreled spline joint showed visible damage.
Another type of flexible coupling in regular use is the diaphragm coupling which transmits torque through a diaphragm of a different diameter to the shafts so that a certain amount of angular or axial displacement can be accommodated.
Flexible torque discs are one type of flexible coupling that can be used to connect two shafts together, transmitting torque from one shaft to the other while allowing small angular and axial displacements of one shaft relative to the other to be accommodated. Typically one shaft is fixed to one side of the torque disc and the other shaft is fixed to the opposite side of the torque disc such that movements (axial or angular) result in deformation of the torque disc.
For reduced weight, torque discs may be formed from composite materials such as carbon fibre reinforced polymer (CFRP). Such composite materials can be designed to have good torque transmission characteristics (i.e. high rigidity) in the rotation direction (i.e. circumferentially) while having a degree of compliance under bending moments (i.e. out of plane loads). WO 2013/064807 describes such a torque disc.
One problem with composite materials is that too high a bending force may cause delamination of layers within the material or may cause small stress fractures that can weaken the disc. Moreover such defects may be difficult to detect (e.g. not visible on visual inspection) and may cause an unacceptable risk of failure.
The bending force that occurs during normal use can be calculated in advance and the joint designed appropriately. However, higher bending moments can occur during installation, maintenance and repair. For example, a shaft on one side of the joint may be displaced by an engineer when effecting a repair or trying to gain access in a confined space. The bending induced by such movement may result in a higher bending moment than the joint was designed for and may result in damage as discussed above.
One way to prevent excessive bending through a coupling is to use hollow shafts for both the driving and driven shafts and to provide a pin across the coupling that passes from the inside of one shaft to the inside of the other shaft. The length and diameter of the pin relative to the internal dimensions of the shafts determines how much angular deviation can take place before the pin engages with the inside walls of the hollow shafts and prevents further bending. However the pin has to span the joint which can make assembly of the joint difficult, particularly in confined spaces.